1. Field of the Invention
The present invention relates to a mobile station position detection method and a system for detecting a position of a person or object who or which carries a mobile station in wireless radiocommunications and a radiocommunication system based thereupon, which are capable of accomplishing the detection of the position of the mobile station with a high accuracy.
2. Description of the Prior Art
In wireless communication fields such as portable radio telephone systems and personal handy-phone systems (PHS), a radio wave strength (an electric field strength) level at reception varies in accordance with the distance from a base station. This signifies that the detection of the strength levels of radio waves picked up from a plurality of base stations in a mobile station side allows the detection of the position of the mobile station, i.e., the position of a person or object who or which carries the mobile station. However, in the case of radio communication systems such as prior portable telephones and PHSs where a plurality of radio zones constitute a service area, the specification of the present location of a mobile station is limited to within a relatively wide area being the range of a radio zone of a base station which accepted the position registration of the mobile station (the range of a general calling area composed of a plurality of radio zones including the radio zone of the base station which accepted the position registration).
For the accurate detection of the location of the mobile station, for example, in the "Mobile Station Position Detection Method" disclosed in Japanese Unexamined Patent Publication No. 2-44929, applying the fact that the radio zones of a plurality of base stations overlap with each other, the present location of the mobile station is specified to a smaller area than the radio zone of one base station on the basis of the mobile station reception radio strength levels from the plurality of base stations and the electric field strength map in the radio zone of each of the base stations. The outline of the typical arrangement of the position detection method disclosed in Japanese Unexamined Patent Publication No. 2-44929 will be described hereinbelow with reference to FIG. 20.
A radiocommunication system, introducing this prior mobile station position detection method, as shown in FIG. 20, comprises base stations 2802, 2803, 2804 respectively having radio zones 2805, 2806, 2807, a mobile station 2801 existing in these radio zones, a mobile communication control station 2808 for taking charge of control of communications between the base stations or between the base stations and a wire network, and a position information center 2809 for detecting the position of the mobile station 2801 on the basis of the information attainable through the mobile communication control station 2808. The positional information center 2809 is composed of a positional information transmission and reception unit 2810 for collecting information about reception radio strengths of the respective base stations 2802, 2803, 2804 and an electric field strength map 2811 indicative of the electric field strength distribution of the respective radio zones 2805, 2806, 2807.
The mobile station 2801 stands in a spot at which the radio zones 2805, 2806, 2807 of the plurality of base stations 2802, 2803, 2804 overlap with each other and, hence, can receive radio waves transmitted from the base stations 2802, 2803, 2804. If the mobile station 2801 is not in a connected communication state with a specific base station, the mobile station 2801 is capable of receiving radio signals (for example, information including identifiers of the base stations, intermittently transmitted through control channels) from these base stations 2802, 2803, 2804.
When receiving the radio signals transmitted from the base stations 2802, 2803, 2804, the mobile station 2801 measures the strength levels of these radio signals and transmits the reception radio strengths on the respective base stations 2802, 2803, 2804, together with the base station identifiers, through any one of the base stations 2802, 2803, 2804 to the mobile communication control station 2808. The mobile communication control station 2808 communicates the reception radio strengths on the base stations 2802, 2803, 2804 from the mobile station 2801, toward the positional information transmission and reception unit 2810 of the positional information center 2809. The positional information center 2810 stores the electric field strength map 2811 made in advance and representative of the electric field strength distribution in the radio zones of all the base stations which, the mobile communication control station 2808 manages, in the form of field strength contours. In the positional information transmission and reception unit 2810, on the basis of the electric field strength map 2811 on the base stations 2802, 2803, 2804, the field strength contours corresponding to the reception radio strength levels on the base stations 2802, 2803, 2804 from the mobile station 2810 are drawn to attain an area in which the respective field strength contours intersect with each other. This area is a spot detected as the position at which the mobile station exists. Thus, the position detection method disclosed in Japanese Unexamined Patent Publication No. 2-44929 can specify the position of the mobile station to within a smaller range than the radio zone of one base station, thereby realizing a position detection with a high accuracy.
However, for applying the position detection method disclosed in Japanese Unexamined Patent Publication No. 2-44929, it is necessary that the electric field strength map on the base stations constituting a service area is known in advance. In general, the electric field strength distribution on some base station is affected by various obstacles or reflectors (roads, houses, buildings and others in the outdoor places, and floors, ceilings, partitions and other in the indoor places) which exist within its radio zone, great difficulty is encountered to predict this electric field strength map from the radio wave characteristic of the base station, and therefore, detailed measurements using a field strength meter or the like are required in order to find the electric field strength map at every base station. In fact, for example, in FIG. 4 (p. 773 (61)) of "Arrangement of PHS Terminals" written by Nakamura, Akazawa, Oka and Mizutori in the Document "NTT R&D" No. 9 (Vol. 44) 1995, pp. 769 (57)-pp. 774(62), there has been shown an electric field strength distribution on a PHS base station in a house. Referring to this, the PHS electric field strength distribution obviously assumes an extremely complicated configuration in a room.
For this reason, for improving the position detection accuracy in this method, a more accurate electric field strength map is required to be obtained through measurements. However, the measurements of the electric field strength distributions on all the base stations require exceedingly expensive labor. Accordingly, the detection of the position of the mobile station with a higher accuracy than that due to the radio zone of one base station is approximately impossible or extremely difficult if taking into consideration the exceedingly expensive labor required for the production of the electric field strength map.
Moreover, for eliminating the above-mentioned problem, the report "Study on Mobile Station Position Detection Based upon Reception Level Information" Electronic Information Communication Scientific Society Autumn Meeting B-269 (1993) presents a method to specify the present location of a mobile station to within a range smaller than a radio zone of one base station through the use of mapping table of reception radio strength levels from a plurality of base stations received by the mobile station and a position (X, Y) of the mobile station by using the fact that the radio zones of the plurality of base stations overlap with each other. This position detection method will be described hereinbelow with reference to FIG. 21. For the basic data for the position detection, reception radio strength levels from a plurality of base stations are measured at points within a service area, and the positions (X, Y) of the measuring points and the radio strength levels (E1, E2, E3, E4 and E5) from base stations BS1, BS2, BS3, BS4 and BS5 received there are made to correspond to each other and accumulated in a database in a center processing section in advance. In the case of detecting the position of the mobile station, radio strength levels (E1', E2', E3', E4' and E5') received from the plurality of base stations are transmitted to a center to be checked with the closest radio strength levels of the radio strength levels accumulated in the database of the center to estimate the position (X', Y') of the mobile station. Thus, the position detection method described in the report "Study on Mobile Station Position Detection Based upon Reception Level Information" can estimate the position of the mobile station within a range smaller than the radio zone of one base station.
There is a problem which arises with this position detection method, however, in that since the estimation of the position is made in the manner that the closest reception radio strength data is retrieved in the position database, the estimated position is limited to the actual measuring points. In addition, in general the recognition of a place is made with a floor or room number in the case of an indoor place and with a building or inherent area name in the case of an outdoor place, and the recognition using the coordinates is inconvenient in many cases. Accordingly, in the case of detecting the position expressed with such a discrete value, it is considered that this position detection method is made such that, for example, the correspondence between room numbers and coordinates is prepared in advance and the estimated position of the mobile station is converted into a room number. However, the the position estimated in terms of a room greatly differs from the position estimated with respect to the vicinity of the boundary between rooms, and hence, even if the closest point is specified on the reception radio strength levels, the error rate increases so that difficulty is encountered to know the degree of the reliability of the position detection.